Vita blue: A new 633‐nm excitable fluorescent dye for cell analysis

Several new derivatives of fluorescein were synthesized. The dyes were characterized by NMR; and the absorbance, excitation, and emission spectra were measured. The fluorescence quantum yields of the dyes were determinde. The pKa3 values of the dyes were measured by fluorescence titration. The characteristics of the fluorescein and sulfonefluorescein derivatives were compared. The most promising dye for use in cell analysis appeared to be compound 9, which was given the name Vita Blue. The dibutyrate ester of Vita Blue was made and the compound was given the name Vita Blue dibutyrate (VBDB, 14). The Km of VBDB with pig liver esterase was measured and found to be 4 × 10−5 M. The pKa3 of Vita Blue was 7.56 ± 0.03; both acidic and basic forms were fluorescent (dual fluorescence). The use of VBDB as an intramolecular esterase substrate and its utility for the discrimination between live and dead cells by flow cytometry is described.